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Why does secondary treatment help protect oxygen levels in rivers and lakes?

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Aerial view of wastewater treatment basins with circular clarifiers; overlaid toilet graphic and arrow with text reading: “Where does it go?” — wastewater treatment All the sewers in your neighborhood end up here.

Secondary treatment protects oxygen levels in rivers and lakes by removing soluble organic matter and suspended solids from wastewater before it is discharged.1

When untreated or poorly treated sewage enters a body of water, microorganisms naturally begin to decompose the organic matter. This biological process consumes dissolved oxygen, a phenomenon measured as biochemical oxygen demand (BOD). If the BOD is too high, the oxygen levels in the water can be depleted, endangering fish and other aquatic life that rely on oxygen to survive.23

During secondary treatment, a wastewater treatment plant provides a controlled environment where microbes consume these organic impurities as food, converting them into carbon dioxide, water, and energy. By significantly reducing the amount of organic matter in the effluent, secondary treatment lowers the BOD of the wastewater, thereby preventing the severe oxygen depletion that would otherwise occur in receiving streams, rivers, or lakes.12